1. Field
The present invention is in the technical field of Audio Signal Processing and, more specifically, to processing of audio signals to compensate for hearing variations of users' hearing.
2. Related Arts
An audiogram is a graph that shows the audible threshold for standardized frequencies as measured by an audiometer. The Y axis represents intensity measured in decibels and the X axis represents frequency measured in Hertz. The threshold of hearing is plotted relative to a standardized curve that represents ‘normal’ hearing, in dB (HL).
Hearing thresholds of humans and other mammals can be found by using behavioural hearing tests or physiological tests. An audiogram can be obtained using a behavioural hearing test called Audiometry. For humans the test involves different tones being presented at a specific frequency (pitch) and intensity (loudness). When the person hears the sound they raise their hand or press a button so that the tester knows that they have heard it. The lowest intensity sound they can hear is recorded.
Ideally the audiogram would show a straight line, but in practice everyone is slightly different, and small variations are considered normal. Larger variations, especially below the norm, may indicate hearing impairment which occurs to some extent with increasing age, but may be exacerbated by prolonged exposure to fairly high noise levels such as by living close to an airport or busy road, work related exposure to high noise, or brief exposure to very high sound levels such as gunshot or music in either a loud band or clubs and pubs. Hearing impairment may also be the result of certain diseases such as CMV or Meniere's disease and these can be diagnosed from the shape of the audiogram.
Aside from hearing impairment, even the most expensive speakers or personal listening devices, such as, e.g., hi-fi stereo headphones with nearly perfect dynamic and spectral frequency characteristics, will produce very different results from one individual to another, depending on their respective hearing abilities. The existing amplifiers or media players are not equipped to provide a listening experience compensated for the listener's specific hearing limitations. The user can typically adjust only the low and high frequency signal levels of the playback device (i.e., bass and treble control) and, in more sophisticated players a simple equalizer can be used to reshape the audio signal according to the listener's personal taste and subjective “feeling”. There are several key disadvantages to any of the above methods.
Generally, the user does not know how to tune each control, as the user does not know the details of his/her hearing characteristics. While hearing sensitivity frequently starts degrading only at high or low frequencies, notches (loss of sensitivity in a narrow spectral range) and un-equal hearing characteristic in both ears are quite common among all age groups. An example of such a hearing characteristic is illustrated in an audiogram in FIG. 1, showing the left and right full spectrum hearing profile. In the diagram of FIG. 1, the sensitivity loss in dB (Y axis) which is plotted as a function of test signal frequency in KHz (X axis) can be easily identified.
Additionally, the adjustment range of prior art equalizers in playback devices is very limited in terms of frequency range, number and width of adjustable bands, and volume levels. Similarly, prior art playback devices do not allow for a dedicated setup per each ear, except for the use of the balance function and a relative (left-right channel) adjustment of volume.
Whereas the commonly used sound equalizers can to a degree compensate for some of the hearing sensitivity loss, it is still true that listening to music from playback devices even through high quality headphones can be dissatisfying and frustrating for users with hearing limitations. To compensate for the reduced ability to hear instrumental and vocal sounds (frequently in a narrow spectral range or partially in one ear), many consumers listen to music at a greatly increased volume, inadvertently risking further hearing degradation. A personalized spectral compensation of the audio signal that counters the specific hearing limitations only within the affected frequency bands offers a much more effective and safer approach to dealing with a moderate hearing degradation and results in a greatly improved listening experience.
It has been well documented in various medical studies (e.g. “Screening and Management of Adult Hearing Loss in Primary Care”, Scientific Review, The Journal of the American Medical Association, Bevan Yueh et al., 2003; 289(15):1976-1985), that person's hearing ability degrades gradually with age. Notably, most people age 35 and up exhibit some degree of natural hearing degradation, which can be responsible for reduced enjoyment of music. Moreover, recent studies (e.g. The Journal of the American Medical Association, Vol. 304, No. 7, 2010) have shown that one in five teenagers in the United States suffers from hearing degradation believed to be caused by proliferation and improper use of personal playback devices. Individual's hearing ability may vary across many variables, including hearing thresholds and noise ambient, sensitivity to specific sounds, dynamic response to loud signals, physical nature of hearing impairment, and psycho-acoustical factors such as e.g. context of the sound. The hearing loss mechanism can be conductive (caused by problems in the outer or middle ear), sensorineural (caused by problems in the cochlea), or neural—caused by a problem in the auditory nerve or auditory pathways/cortex of the brain.
In many situations for hearing impaired an individually tailored hearing aid is frequently the best solution. The process of selecting the hearing aid entails detailed testing in an audiologist office using sophisticated equipment and highly trained personnel. In a typical audiogram the hearing sensitivity response is measured using limited four to eight basic frequency bands with the focus on understanding of human speech in noisy environments. Filtering out the noise from the background and/or selectively amplifying signal-of-interest is of primary importance to hearing aid design. Consequently, hearing aids are typically good at correcting a relatively narrow frequency range with focus on 3 KHz-4 KHz band corresponding to the human voice, while an average human auditory range extends from 20 Hz to beyond 16 kHz. Moreover, hearing aid devices are inherently different from playback devices because they are built for use in an open-air environment and can't be used to listen to music together with headphones or earphones.
The rise of the Internet has opened possibility for various on-line hearing tests and development of personalization techniques taking into account individuals hearing profiles. Number of testing regimes and methods have been offered in recent years, however, few were successful at providing an effective, automated sound personalization that could be practically implemented in common playback devices in everyday use. The various sound enhancement and compensation methods commonly used in today's playback devices include passive noise attenuation, selective amplification of the signal of interest, statistical signal filtering, automatic gain control (AGC), active noise cancelation, or any combination of the above.
In a recent patent (U.S. Pat. No. 8,112,166) by Pavlovic et al., the authors conclude that the efforts in the art have not succeeded in providing a system for effectively and rapidly generating individual's hearing profile, quote: “The most widespread employment of individual hearing profiles remains in the hearing aid field, where some degree of hearing impairment makes intervention a necessity.”
In another patent example (U.S. Pat. Nos. 6,944,474 and 7,529,545) Rader et al., attempted methods of personalizing audio signals based on individual's hearing profile, personal preferences, and environmental noise factors. In this example a personal communication device such as e.g. smart phone comprising a control circuitry and logic, is used to apply multiband compression to the audio signals using the parameters derived from standard, age dependent, externally stored hearing profiles. In this approach, the signal effects are adjusted based among others on perceived sensitivity of the ears of the individual. However, the inventors don't teach the exact methods of measuring the hearing impairment nor do they explain how the elevated threshold of audibility of a tone due to impairment at a given frequency is used to modify the full frequency dynamic compression (FFDC) algorithm and the corresponding automatic gain control (AGC) proposed in the patent. The authors aspire to replicate the function of the healthy cochlea by compressing the sound such that the audio signal is no longer distorted in the auditory system. This approach has been proposed in monaural Bluetooth listening devices, which are not commonly used for listening to music.
In another set of patent publications (US 2014/0334644 A1, US 2014/0309549 A1 and US 2014/0314261 A1) Selig et al., describes several “Methods for Augmenting Hearing”. In one variation, a mobile device is outputting a tone in its hearing test. Based on the user response to the tone, the method qualifies the signal as a particular audio type (based on pre-defined profiles), and process the audio signal based on that profile. A variation of this method selects a suitable profile based on the location of the audio device. Another variation tries to select from a pre-defined hearing models by outputting two tones to the user and recording the user's volume adjustment. These methods have a major limitation in the fact that they try to best match the user's hearing profile to a set of pre-defined and stored profile. While this method can provide audible results, it can't provide a perfect match to the user's hearing profile and deficiencies, and hence cannot truly compensate for all of the user's specific hearing deficiencies, nor to the dynamic and the frequency response of the playback devices used.